Fix calling gpm_inquire_cred_by_mech
    
    We need to pass the real mechanism oid here, not the spcial oid.
    special oids are used exclusively by the interposer and gssapi
    machinery that calls the interposer, they must never be propagated
    to clients or servers.
    
    https://fedorahosted.org/gss-proxy/ticket/107
    
    Reviewed-by: Günther Deschner <gdeschner@redhat.com>